1. Field of the Invention
This invention relates generally to cell populations derived from neurons, which are denoted neuroblasts, methods for the production and long-term in vitro culture of these cell populations, and the use of neuroblasts in the treatment of various neuronal disorders as well as the identification of compositions which affect neuroblasts.
2. Description of Related Art
Only a few neuronal cell types have been reported to divide in the adult brain and adult neurons do not survive well in vitro. To date, even with all of the recent advances in neurobiology, genetics, immunology and molecular biology, no reliable procedure exists to establish cell lines from the central nervous system (CNS) and neuronal tissues in the absence of immortalization. The generation of clonal cell lines from different regions of the brain is important and will greatly facilitate the discovery of new neurotrophic factors and their receptors, and enhance the understanding of their function.
The central nervous system contains two major classes of cells known as neurons and glial cells. Glial cells include astrocytes, oligodendrocytes and microglia. There are hundreds of different types of neurons and many different neurotrophic factors which influence their growth and differentiation. Depending on the type of neuron and the region of the brain in which the neuron resides, a different neurotrophic factor or specific combination of factors affect the survival, proliferation and differentation of the neuron. Each type of neuron responds to different combinations of neurotransmitters, neurotrophic factors, and other molecules in its environment.
To date, neuropharmacological studies in the CNS have been delayed by the lack of cell systems needed to investigate potentially useful neuroactive compounds. In live animals, the complexity of the brain makes it difficult to effectively measure which cellular receptors are being targeted by these compounds. Additionally, the expense involved in live animal research and the current controversies stemming from animal rights movements have made in vivo animal studies less acceptable for initial research. Primary cells from neuronal tissue are often used for CNS studies, however, long-term culture of primary neurons has not been achieved. Also, only a few attempts to achieve not only long term culture, but actual proliferation of neuronal cells have been reported. In fact, the proliferation of neuronal cells has proven so elusive that it has become ingrained in the scientific community that neuronal cells do not proliferate in vitro. As a consequence, fresh dissections must be performed for each study in order to obtain the necessary neuronal cell types, resulting in costly research with increased variability in the experimental results.
While some neuronal tumorogenic cells exist they are few in number and are not well characterized. In general, these tumor call lines do not mimic the biology of the primary neurons from which they were originally established and, as a result, are not suitable for drug discovery screening programs. In vitro primary cultures that would be more phenotypically representative of primary cells and that could generate continuous cultures of specific neuronal cell lines capable of proliferation would be invaluable for neurobiological studies and CNS drug discovery efforts, as well as therapy.
It has become increasingly apparent that more defined conditions and further refinements in culture methodology are necessary to produce neuronal cell lines which would enhance the yield of information from in vitro studies of the nervous system. Recognition of cell type and developmental stage-specific requirements for maintaining neural cells in culture as well as the development of a broader range of culture conditions are required. However, in order to achieve these goals it is critical to develop optimal culture methods which mimic in vivo conditions which are devoid of the biological fluids used in conventional culture techniques.
Recently, several researchers have isolated and immortalized progenitor cells from various regions of the brain and different stages of development. Olfactory and cerebellum cells have been immortalized using the viral myc (v-myc) oncogene to generate cell lines with neuronal and glial phenotypes (Ryder, et al., J. Neurobiology, 21:356, 1990). Similar studies by Snyder, et al. (Cell, 68:33, 1992) resulted in multipotent neuronal cell lines which were engrafted into the rat cerebellum to form neurons and glial cells. In other studies, murine neuroepithelial cells were immortalized with a retrovirus vector containing c-myc and were cultured with growth factors to form differentiated cell types similar to astrocytes and neurons (Barlett, et al., Proc. Natl. Acad. Sci. USA, 85:3255, 1988).
Epidermal growth factor (EGF) has been used to induce the in vitro proliferation of a small number of cells isolated from the striatum of the adult mouse brain (Reynolds and Weiss, Science, 255:1707 1992). Clusters of these cells had antigenic properties of neuroepithelial stem calls and under appropriate conditions, these cells could be induced to differentiate into astrocytes and neurons with phenotypes characteristic of the adult striatum in vivo. However, it should be noted that these differentiated neurons were not cultured for lengthy periods of time nor was there any evidence that these cells could be frozen and then thawed and recultured.
Cattaneo and McKay (Nature 347:762, 1990) performed experiments using rat striatum to determine the effect of nerve growth factor (NGF) on proliferation of neuronal precursor cells. The cells were dissected from rat embryonic striatum and exposed to both NGF and basic fibroblast growth factor (bFGF, also known as FGF2). These cells were cultured only nine days in vitro, at which time they had differentiated into neurons as determined by assay with neuron-specific markers.
Neuronal precursor cells from the cerebral hemispheres of 13-day old rat embryos have been cultured for up to 8 days in the presence, of bFGF at 5 ng/ml (Gensberger, et al., FEBS Lett. 217:1, 1987. At this concentration, bFGF stimulated only short-term proliferation. Proliferation and differentiation of primary neurons from both fetal and adult striatum in response to a combination of NGF and bFGF or only EGF have also been reported (Catteneo, et al. supra; Reynolds and Weiss, supra).
In view of the foregoing, there is a need for a long-term in vitro culture system which would allow large scale production and maintenance of a neuronal cell population which will proliferate and can be passaged and subcultured over time. Such homogenous in vitro neuronal cultures will prove invaluable in studying cell populations, the interactions between these cells and the effects of various neuroactive compositions on these cells.
Recognizing the importance of a system for producing and maintaining neuronal cells in vitro, the inventors developed a method and a culture system for producing continuous fetal and adult neuronal cell lines. The development of primary neuronal cultures maintained as cell lines, known as neuroblasts, using neurotrophic factors in the absence of oncogenic immortalization, now permits investigation of fundamental questions regarding the biochemical and cellular properties of these cells and the dynamics of interaction between their cellular and chemical environment.
The neuroblasts of the invention can advantageously be used to stably incorporate genetic sequences encoding various receptors, ligands and neurotransmitters, for example, for use in the treatment of subjects with neuronal disorders and for identifying compositions which interact with these molecules.